1. Field of the Invention
The present invention relates to a stepwise automatic transmission for a straddle-type vehicle, a power unit including the stepwise automatic transmission for the straddle-type vehicle, and a straddle-type vehicle including the power unit.
2. Description of the Related Art
Conventionally, a stepwise automatic transmission is known. The stepwise automatic transmission generally has higher energy transmitting efficiency than that of a belt type continuously variable transmission. Therefore, the need for stepwise automatic transmissions has been increased in recent years.
For example, Japanese Utility Model Application Laid-Open No. S62-23349 discloses a three-speed stepwise automatic transmission. FIG. 14 is a sectional view of the three-speed stepwise automatic transmission 200 disclosed in the Japanese Utility Model Application Laid-Open No. S62-23349. As shown in FIG. 14, the stepwise automatic transmission 200 includes an automatic centrifugal type start clutch 202 using a planetary gear provided on a left side of a crankshaft 201, and a drive sprocket 203. Rotation of the crankshaft 201 is transmitted to a drive sprocket 203 through the automatic centrifugal type start clutch 202.
A main shaft 204 is disposed behind the crankshaft 201. A driven sprocket 205 is mounted on the main shaft 204. A chain 206 is wound around between the driven sprocket 205 and the drive sprocket 203.
An automatic centrifugal type high speed clutch 207 is disposed on the main shaft 204. In the stepwise automatic transmission 200, gears are shifted between the second speed and the third speed by the automatic centrifugal type high speed clutch 207.
According to the Japanese Utility Model Application Laid-Open No. S62-23349, the automatic centrifugal type start clutch 202 and the automatic centrifugal type high speed clutch 207 are disposed in a longitudinal direction. With this arrangement, a relatively narrow stepwise automatic transmission is achieved.
As shown in FIG. 14, in the stepwise automatic transmission disclosed in the Japanese Utility Model Application Laid-Open No. S62-23349, the automatic centrifugal type start clutch 202 and the automatic centrifugal type high speed clutch 207 which are heavy loads are disposed at a large distance therebetween in the longitudinal direction. Therefore, there is a problem that it is difficult to concentrate a longitudinal mass.